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A student creates a 1M citrate (C3H5O(COO)33-) buffer solution at pH 5.0. He then dilutes the solution to 200 mM, and adds it to his experiment. What is the pH of the 200 mM buffer solution?

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Correct Answer: D. The pH will remain close to 5.
Buffers are designed to maintain pH at a certain level, and operate through the complementation of a conjugate acid and base. As long as the ratios of acid and base remain the same in the buffer system, and they will no matter what the dilution, as long as the dilution is not very very small (i.e. 1 mM or less), the pH will remain the same.
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